Natural rubber latex gloves provide excellent protection from numerous dangerous pathogens as well as many harsh chemicals. The natural rubber latex glove manufacturing industry mushroomed in early 1980s, especially in the Far East. However, soon after that, it was recognized that the inherent proteins of natural rubber latex would cause allergic reactions (Type I) to occur in certain people. In rare cases, the allergic reaction could be fatal. Therefore, for those people, alternatives, to natural rubber latex gloves, must be provided.
Although a series of synthetic materials including nitrile butadiene rubber (NBR), polychloroprene (CR), polyurethane (PU), polyisoprene (IR), polyvinyl chloride (vinyl, PVC), polyethylene (PE), etc. as well as many of their blends and copolymers have been used as alternatives to natural rubber latex, the overall performance and the cost of the alternatives are not quite satisfactory. Among the alternatives, nitrile butadiene rubber is the most popular one, an elastic glove at a reasonable cost.
U.S. Pat. No 5,014,312, and Reissue Patent RE 35,616, both issued to Tillotson et al, cover nitrile butadiene rubber gloves. The patents address relaxation properties. The stress (or modulus) of the material under constant strain at six minutes should be less than 50% of its initial value. Most of the nitrile gloves currently commercially available have their relaxation property clustered about 40%, although that could be varying from 30˜45%. Other gloves might have improved tensile strength, or elongation, or fewer additives that could cause Type IV allergic reactions (ZnO, etc.). None of them have displayed relaxations at six minutes that could exceed 50%.
Tillotson et al compared relaxation properties between natural rubber latex and nitrile films, but has not shown that the relaxation property of nitrile films can be tuned within a certain range via proper formulation.
U.S. Pat. No. 6,031,042 issued to Lipinski reveals a nitrile rubber formulation. The formulation contains no zinc oxide and only 1.0 PHR sulfur resulting in a relaxation property of only approximately 40%. No consideration of adjusting the formulation to tune the relaxation property of the glove is contemplated.
U.S. Pat. No. 6,566,435 issued to Teoh et al discloses a nitrile latex formulation containing less than 0.5 PHR zinc oxide and sulfur. No contemplation is given to a product with a zinc oxide content of greater than 0.5 parts per hundred and less than 1.0 parts per hundred with a relation property tuned to above 50%. More over, the proposed formula restricted the latex to be used. It must consist of a carboxylated content between 2˜6%. We found the content of the carboxylated group is not as critical as they claimed. Conversely, we used a product that contains 7% of carboxylated group and the relaxation property can still be much higher than 50%. We also tested the sample formula in this patent to make some films. The films do not age well.
Relaxation property is not an ASTM required quality control parameter for gloves. But together with modulus, another non-ASTM required quality control parameter for gloves; they can characterize the performance and the tactile sensation of a glove. The higher the relaxation property, the better the glove will fit a hand's shape. Otherwise, the glove becomes loose after a while. But if high relaxation were combined with high modulus, the glove would quickly cause finger fatigue. Natural rubber latex gloves has a (relaxation >80%, and a 300% modulus (<2 MPa), while nitrile butadiene rubber gloves show lower relaxation (typical 40%) and a much higher modulus at 300% (>7 Mpa).
Relaxation property is an intrinsic characteristic of material nature. Most nitrile butadiene latexes manufactured via emulsion polymerization would yield a relaxation of about 40%, as evidenced by the nitrile gloves currently available. This inherent property is predominantly caused by polymer chain structure, which would be determined by the polymerization mechanism. Different nitrile butadiene rubber vendors might have different controlling parameters and procedures, but their products have very little differentiation due to the fact that they all use emulsion polymerization for economic reasons.
Nitrile butadiene latexes, produced via polymerization mechanisms other than emulsification, namely for dipping applications could have quite different structure, and thus different relaxation profiles, but there are no such products that are commercially available right now because of cost. Once the polymer chain structure has been predefined in the polymerization, there is little one can do to manipulate it. It is an objective of the invention to tune this parameter (relaxation) to above 50%. Meanwhile, the other mechanical properties must meet ASTM requirements.
To evaluate the performance of a nitrile glove, relaxation is only one of many physical properties. Tensile strength, modulus, elongation, before and after aging, are all very important; as required by ASTM. Gloves depending on the designed application can emphasize different characteristics.
Three components in formulation are critical to affect these desired properties. First of all, sulfur is the crosslinker. Secondly, zinc oxide is the so-called primary activator for sulfur vulcanization. Lastly, the so-called secondary accelerators include zinc dibutyldithiocarbamate (ZDBC or BZ), zinc diethyldithiocarbamate (ZDEC or EZ), zinc 2-mercaptobenzothiazole (ZMBT), etc.
Through experimentation, we find out that ionic crosslinking will lead to strong tensile strength and good aging resistance, as desired. However, it also results in low elongation, high modulus, and low relaxation, as undesired. Zinc oxide free would result in highest relaxation, lowest modulus, and longest elongation. But the films don't age well. The films show fairly poor aged elongation (<400%).
Sulfur content is critical to relaxation. We made two series of films with sulfur content 1 PHR and 3 PHR but changed the zinc oxide from 1 PHR to 0. The correlation is clear. At 1 PHR of sulfur, relaxation would go from 35% to 45% by decreasing zinc oxide from 1 PHR to 0. At 3 PHR of sulfur, relaxation would go from 45% to 55%.
Having reduced zinc oxide, usually one has to add some more secondary accelerators to compensate, so that sulfur vulcanization can occur timely as desired. However, excessive secondary accelerators not only cause concerns on Type IV allergy, but also accelerate aging of the products during storage.
Varying the ratio of these three components, one can have optimized formulations for different desired applications.